1. Field of the Invention
The present invention relates to a method to determine operating conditions of a wavelength tunable laser diode (hereafter denoted as a tunable LD) and to control an optical transmitter implementing the tunable LD.
2. Background Arts
One type of diffraction gratings has been well known in the field as the sampled grating where a plurality of segments is continuously formed, each of segments includes a diffraction grating and a space with no diffraction grating formed continuously next to the diffraction grating. The sampled grating inherently has a function to select a wavelength. For instance, a sampled grating with an optical gain, which is called as the SG-DFB (sampled grating distributed feedback), may be integrated with a sampled grating without any optical gain but showing wavelength selective reflectivity to select one specific wavelength, which is called as the SG-DBR (sampled grating distributed Bragg reflector). Various prior arts have disclosed a tunable LD having the SG-DFB and the SG-DBR. In particular, one type of the SG-DBRs including a chirped sampled grating distributed Bragg reflector, which is called as CSG-DBR, combined with the SG-DFB has been well known in the field.
In the SG-DFB and/or the SG-DBR, the diffraction gratings in respective segments are formed so as to align the phases thereof. However, some reasons, for instance, the instability of the manufacturing process of the diffraction grating and/or the stress induced in the segments, sometimes disorder the phase of the diffraction grating in respective segments within dimensions less than a period of the diffraction gratings. Another reason such as inhomogeneous distribution of the temperature and/or the temperature deviated from the designed one cause undesired variation of the equivalent refractive index in the segment, which also disorders the optical interaction between the SG-DFB and the SG-DBR. The present application is to provide a solution to compensate such deviation.